1. Field of the Invention
The invention relates to alkoxy-crosslinking RTV1 silicone rubber mixtures based on hydroxy-terminated polyorganosiloxanes containing a trialkoxysilane with a substituted amidoalkyl radical.
2. Discussion of the Background
One-component silicone rubber mixtures (RTV1) which can be stored if water is excluded and which in the presence of water vulcanize at room temperature to give elastomers are known. The vulcanization of the mixtures may take place under conditions which are acidic, e.g. in the presence of acetoxysilanes, basic, e.g. using aminosilanes, or neutral, e.g. by means of compounds which have oximo or alkoxy groups. RTV1 systems which crosslink under neutral conditions are needed especially in circumstances where the elimination products produced when the mixtures are cured should not affect the substrate, for example in the jointing of concrete or of metallic materials.
RTV1 materials containing acetate-, amine- and oxime-crosslinking agents are typically prepared by mixing an OH-terminated polydimethylsiloxane with a plasticizer, which is usually a methyl-terminated polydimethyl-siloxane, an acetoxy-, amino- or oximosilane as a crosslinking agent, an active filler, for example fine-particle silica, and a catalyst, which is usually a dialkyltin compound, in the order stated. Additives, such as curing accelerators, pigments, processing aids and other fillers, may be added. Except for a few very specific applications which require flowable compositions, the requirement is mainly for mixtures of firm consistency which contain only fine-particle silica as filler. The mixing systems used for RTV1 materials are usually simple planetary mixers or dissolvers. There are also processes which allow continuous preparation of the mixture, e.g. using extruders. In the case of alkoxy-crosslinking RTV1 compositions, however, it is not possible to use OH-terminated polydimethylsiloxane other than in a few exceptional cases. If the components are mixed in the manner described above, using an alkoxysilane as crosslinking agent, vulcanization begins as early as during the preparation of the mixture. For this type of RTV1 mixture, a polymer is first prepared separately in a preliminary step by reacting hydroxy-terminated polydiorganosiloxanes with alkoxysilanes in the presence of catalysts (EP 1376 883, EP 304 701, EP 559 045 and many others). The alkoxysilane used is usually methyltrimethoxysilane. There are also known processes in which compounds such as aminopropyltrimethoxysiloxane, aminoethylaminopropyltrimethoxysilane, glycidoxypropyltrimethoxysilane and methacryloxypropyltrimethoxysilane are employed (DE 44 05 851). The preparation of the mixture then follows the preparation of the alkoxy-endgroup polymer, and takes place as described above. The separate preparation of the specific polymer, however, is inconvenient and requires additional expense.
Attempts are therefor being made to use the readily obtainable hydroxy-terminated polydiorganosiloxanes directly as polymers in the preparation of alkoxy-crosslinking RTV1 compositions. For example, EP 384 609 and EP 543 615 describe methoxy-crosslinking compositions which are mixed with a very large amount of filler and have been prepared using OH-terminated polydimethylsiloxanes. However, this method functions only where the polymer, the plasticizer and the fillers (in this case a combination of precipitated and ground chalk) are first mixed with one another, and then methyltrimethoxy-siloxane, as a crosslinking agent, and a crosslinking catalyst are added afterward. This initially gives a material which has an extremely high viscosity and may even be to some extent elastomeric, and which can only be processed using specific mixing technologies. The mixing technology normally used for RTV1 compositions, e.g. planetary mixers and dissolvers, cannot be used. In addition, this procedure is limited to highly filled methoxy-crosslinking compositions. If the ethoxysilanes which have been described hitherto are used, premature crosslinking takes place as early as during the mixing or during the storage of the finished pastes and thus the products obtained are not sufficiently storage-stable. For transparent systems filled only with fine-particle silica, this method cannot be used at all. It is known that mixing of OH-end-group polymer with silica, without a crosslinking agent, gives a very highly viscous paste, which rapidly stiffens to give a hard crumbly material. When a crosslinking agent is added to a mixture of this type, the firm consistency of the paste is lost. However it is just such firm-consistency compositions which are needed. In addition, the premature crosslinking of compositions prepared in accordance with the prior art described above cannot be controlled.
Because of the toxicity of methanol, it is moreover desirable to be able to prepare alkoxy-crosslinking RTV1 compositions which produce only non-hazardous elimination products, e.g. ethanol.